Yes in fact they do, the mattress industry uses: 2 twin E.L. (Extra Long) boxsprings which each measures 38 x 80 each. The twin E.L box springs are then placed underneath of a 76 x 80 eastern king mattress.
Instant King Twin Bed Bridge. There's no reason to buy a costly king size bed, when you can simply convert two twins to meet your needs. Made of soft and supportive hypo-allergenic polyurethane foam, this bed bridge bridges the gap, leaving no ridges or lumps.
Make 1 King Size Bed with two Twin Size Beds. Place the Bed Connect over the seam between the mattresses. Make one larger bed out of two when you need it .
There is a funny-looking gadget on sale in several places that can be used to fill the gap between the two joined beds. This makes things a bit more comfortable .
Some people push two twin beds together in the hopes of making one larger bed. This can work for something things. It's especially good if there are two people .